The question of the role of the glial cells in the clearance of a local accumulation of potassium in extracellular space will be studied in a model preparation, the retina of the honey bee drone. In this preparation it has been shown with intracellular potassium-sensitive microelectrodes that light stimulation causes changes in the concentration of potassium in the photoreceptors, the extracellular space and the glial cells. The ion movements concommitant with the entry of potassium into the glial cells will be investigated with the aid of intra and extra-cellular microelectrodes sensitive to potassium, sodium and chloride. The relative importance of space-dependent and space-independent mechanisms of potassium clearance will be elucidated by altering the area of the light stimulus and the volume of tissue that is oxygenated, by passing electric currents through the retina, and by changing the composition of the superfusate.